194 



mination of the nasals rather exceeds that of the cranial part at the parietal region; it 

 is also greater than the beginning of the facial part of the skull in advance of the orbits 

 and molar alveoli, whence there is a gain both in depth and breadth as it approaches 

 the anterior terminations ; but the uniformity of this diameter of the skull along the 

 medial line, from the superoccipital forward to the premaxillo-nasal part, viewed side- 

 ways or in direct profile, is a remarkable characteristic of Diproto&on. 



The part of the maxillary (Plate XIX. fig. 1, 21) lodging the molar series of teeth 

 ((/ 3-m 3) breaks the lower line of the profile, descending below it along the middle third 

 of the length of the skull. The zygomatic arch is deep, long, but proportionally less con- 

 vex outwardly, or less expanded, than in TJiylacoleo. Its base (ib. figs. 1 & 3, 27) seems as 

 if continued from the whole side of the occipital plane, contracting rapidly at the upper 

 border as the arch sweeps outward and forward ; the superoccipital crest being continued 

 into the upper border of the arch, and this apparently without break or abrupt rise in 

 any part of that border*. The frame of the orifice of the "meatus auditorius" (ib. 

 fig. 1, 2e) projects downward from the hind part of the lower border of the base of the 

 zygoma, indicative of the tympanic. Immediately in front of this descends the postglenoid 

 process (a) of the squamosal, and in advance of this is a second downward projection or 

 convexity due to the " eminentia articularis " (b), which is here, as in Marsupials, a process 

 of the malar (26). From this part the lower border of the zygoma runs forward nearly 

 parallel to the upper one, but with a slight concavity, as far as the maxillary element of 

 the zygoma, which sends down a strong, moderately long, obtuse, subcompressed masse- 

 teric process (ib. 21') — a cranial feature which is peculiar to herbivorous Marsupialsf. 



The orbit (ib. fig. 1, r) is a relatively small vertically oval cavity, communicating 

 widely behind with the temporal fossa (ib. 7). The external nostril (ib. figs. 1 & 2, n) 

 is terminal, subvertical, rather expanded, and divided in great part by an upward exten- 

 sion of the medial nasal plate of each premaxillary (ib. figs. 1 & 2,22'), which plates, 

 being in close contact, form the lower part of a long " septum narium " at the outlet of 

 the nasal cavity, recalling its condition in the extinct Rhinoceros tichorhinus. There 

 is a narrow and short descending ridge at the coadapted medial borders of the nasals, 

 which seems to have been continued into the septum by cartilage rather than by bone. 

 I have alluded to the analogy which the structure of the external nostrils in Biprotodon 

 suggest to those of an extinct Pachyderm, but the truer and closer resemblance is found 

 in the Marsupial group. The cavity of the nose is divided by a complete bony septum 

 to within one-fourth of the outer opening in Macropus and Phascolomys%, advancing, in 

 one species of Wombat, as in Nototherium, nearer to that outlet. 



* ^onic mutilation of the hind part of this upper border in both zygomata begets reserve in definitely pro- 

 nouncing as to its normal outline. 



■r The descending masseteric process in Glyptodonts, Sloths, and Megatherioids is formed by the malar bone 

 exclusively. Owen, ' Anatomy of Vertebrates,' vol. ii. p. 405, figs. 273, 274, 26, a. 



* " On the Osteology of the Marsupialia," Zoological Transactions, vol. ii. p. 391. Anatomy of Vertebrates, 

 vol ii. p. 348. 



